This invention relates to a method for molding a plastic article and, more particularly, to a method for molding a plastic article which uses incremental monitoring of the injection process without having to overpack the mold unit with plastic material.
Injection molding machines generally include a two-section mold unit wherein one of the mold sections is stationary and includes an end gate opening. The opening allows the injection of thermoplastic material into a cavity formed by the mold unit. The other mold section is generally movable between an open position away from the stationary mold section, and a closed position wherein the two mold sections are in sealed contact engagement to form the mold cavity.
Once the mold cavity has been formed, a reciprocating screw or similar injection device is used to inject a plastic material into the mold cavity where the material hardens with time. During injection of the plastic material, the mold unit is typically cooled. The cooling causes the plastic material to harden faster, so that the mold unit can be used to mold a greater number of plastic articles than if the mold unit were not cooled. As the plastic material is injected into the mold cavity, the leading edge of the plastic material forms a melt front which spreads across the mold cavity as the mold cavity fills. The plastic material, however, is typically injected into the mold cavity very quickly under high pressure to prevent the melt front from hardening too quickly and blocking the mold cavity. This high pressure injection breaks the smooth melt front and causes an erratic spray of plastic material into the mold cavity. Accordingly, instead of an unbroken symmetrical melt front evenly filling the mold cavity, the plastic material is sprayed throughout the mold cavity, thereby causing an asymmetrical filling of the cavity. This asymmetrical filling has the undesirable effect of preventing the plastic from filling every portion of the mold cavity. Additionally, as the plastic material is sprayed into the mold cavity, portions of the mold cavity are filled before others, leading to irregular hardening of the plastic material. A further problem with typical high pressure injection mold filling techniques is pressure needed to overcome such erratic filling of the mold cavity. Just as it is difficult to fill a glass of water completely with a high pressure hose, so too is it difficult to completely fill a mold cavity with high pressure injection techniques. As the cavity is filled, the plastic material begins to backflow, causing turbulence and requiring greater clamp pressures and injection pressures.
To overcome the asymmetrical filling and backflow associated with prior art processes, and to assure that the mold cavity becomes completely filled with plastic material, a volume of plastic material greater than the volume of the mold cavity is injected into the mold cavity and maintained under high pressure until the plastic begins to harden. The injection of an excess amount of material into the mold cavity requires a sharp pressure increase or "spike" to pack the material into the mold cavity. While "packing" the mold cavity with an excess of plastic material completely fills the mold cavity during the molding process, the pressure spike causes enormous outward pressure on the mold sections, leading to an increased amount of wear on the mold sections and clamping apparatus. Despite constructing mold units of steel to handle the pressure spikes associated with prior art molding processes, these mold units often wear prematively, requiring the mold units to be returned to the manufacturers for repair. Depending on the severity of the wear and the repair schedule of the manufacturers, worn mold units can be out of service for several weeks. The loss of production associated with the repair of cracked mold units is generally very costly.
The increased outward pressure on the mold sections may even lead to movement of the mold sections away from one another. When the mold sections move apart, plastic material seeps into the parting line between the mold sections. This "flash" is not only aesthetically undesirable on a finished plastic part, but also leads to a waste of material and creates an uneven parting line between the mold sections. As the clamping apparatus presses the mold sections together against the plastic hardening between the parting line, the parting line becomes deformed and uneven, thereby making flash even more likely upon subsequent moldings.
Additional problems associated with prior art high pressure molding techniques are the increased amount of material needed to be placed into the mold cavity, as well as the pressure gradients created throughout the molded part due to the increased amount of material being packed into a finite volume mold cavity. In some cases, the pressure gradient is so large that it leads to warpage of the finished plastic article. If the warpage is great enough, the plastic article may not fit within design tolerances.
While it would be desirable to fill the mold cavity slowly throughout the molding process, to avoid asymmetrical filling and backflow, it is difficult to inject the plastic material slowly without uneven curing and blockage within the cavity. If the mold cavity is filled too slowly, the first portion of the plastic material to be injected into the mold cavity begins to harden as the rest of the plastic material is still being injected. This premature hardening leads to clogging of the mold cavity during the filling process. Such clogging is particularly undesirable since it requires stopping a molding run and discarding the partially hardened materialThe
The difficulties encountered in the prior art discussed hereinabove are substantially eliminated by the present invention.